Method and system for operating an air gauge at programmable or constant standoff

ABSTRACT

Provided are a methods and systems for determining a topography of an object. In an embodiment, a system includes a reference probe configured to measure a surface of a reference surface and to generate a reference signal, a measuring probe configured to measure a surface of an object and to generate a measurement signal, a sensor configured to sense a position of the measuring probe and to generate a sensor signal, and a combiner configured to receive the sensor signal and the measurement signal and to generate a combination signal therefrom. A desired distance between the measuring probe and the object is substantially maintained by adjusting the position of the measuring probe based on the measurement signal. A topography of the object is determined based at least on a comparison of the reference signal and the combination signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Non-Provisional applicationSer. No. 11/011,435, filed Dec. 15, 2004, now allowed, which isincorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to pressure sensors, more particularly, toair gauge devices used in lithography systems.

2. Background Art

Conventional style air gauges are used to measure the location of awafer surface in a number of lithography tools. These conventional airgauges include a bridge having a measurement nozzle located near thewafer's surface. The conventional air gauges typically also include aseparate reference nozzle located near, or in the same environment as,measurement components. As a gap between the wafer and the measurementnozzle changes, the flow rate through the measurement nozzle is altered,and a change in differential pressure or flow in the bridge is detected.

In general, although the measurement nozzle may be retractable, itsposition is fixed during the measurement process. Likewise, the gapbetween a reference nozzle and its target may be adjustable, but remainsfixed during the measurement process. The gap measurements made by theseconventional air gauges are most accurate when the wafer surface is atthe nominal gap where the flow through the bridge is nearly balanced,and becomes less accurate as the measurement gap moves away from thenominal value. Off null, the air gauge becomes sensitive to changes andambient pressure, and the relationship between gap and senseddifferential flow or pressure is non-linear.

The air gauge can be used at typical standoffs of less thanapproximately 0.150 millimeters (mm). At the physical scales of interestto wafer surface sensing, a substantial increase in an air gaugestandoff value (H) is not possible, as the measurement sensitivity dropsquite drastically, approximately to H^(−3.3). At such small standoffs,there is a possibility of a collision between the air gauge nozzle and,for example, a wafer surface. Also, to the extent that the air gauge isrequired to accurately measure a range of wafer positions, its accuracyis limited.

What is needed, therefore, is a method and system for facilitatingmeasurements where the air gauge will always be operated at a favorablestandoff, maximizing its performance and useful measurement range. Morespecifically, what is needed is a gauging device that will minimize therisk of a collision between the air gauge nozzle and the surface of thewafer.

BRIEF SUMMARY OF THE INVENTION

One embodiment of the present invention provides a system that includesa system includes a reference probe configured to measure a surface of areference surface and to generate a reference signal, a measuring probeconfigured to measure a surface of an object and to generate ameasurement signal, a sensor configured to sense a position of themeasuring probe and to generate a sensor signal, and a combinerconfigured to receive the sensor signal and the measurement signal andto generate a combination signal therefrom. A desired distance betweenthe measuring probe and the object is substantially maintained byadjusting the position of the measuring probe based on the measurementsignal. A topography of the object is determined based at least on acomparison of the reference signal and the combination signal.

In a further embodiment, the system further includes an actuatorconfigured adjust the position of the measuring probe so that thedesired distance between the measuring probe and the object issubstantially maintained.

In a further embodiment, the system further includes a controllerconfigured to generate a control signal. The actuator is configured toadjust the position of the measuring probe based on the control signal.

In a further embodiment, the measuring probe is a self-gapping measuringprobe configured to self-adjust its position to substantially maintainthe desired distance between the measuring probe and the object.

In another embodiment, a method includes measuring a distance to areference surface, measuring a distance to an object using a measuringprobe, adjusting a position of the measuring probe used to measure thedistance to the object, such that a desired distance between themeasuring probe and the object is substantially maintained, sensing theposition of the measuring probe, generating a combined signal based onthe measured distance to the object and the sensed position, anddetermining a topography of the object based at least on the combinedsignal.

In a further embodiment, adjusting a position of the measuring probeincludes generating a control signal based on which an actuator isconfigured to adjust the position of the measuring probe.

In a further embodiment, adjusting a position of the measuring probeincludes adjusting at least one of internally produced force of themeasuring probe or a preload force of a spring to adjust the position ofthe measuring probe.

Further features and advantages of the present invention as well as thestructure and operation of various embodiments of the present invention,are described in detail below with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated in and constitute partof the specification, illustrate embodiments of the invention and,together with the general description given above and the detaileddescription of the embodiment given below, serve to explain theprinciples of the present invention. In the drawings:

FIG. 1 is a block diagram illustration of a gas proximity sensingapparatus;

FIG. 2 is a block diagram illustration of a gauging device constructedin accordance with an embodiment of the present invention and used inthe apparatus of FIG. 1;

FIG. 3 is a block diagram illustration of a gauging apparatusconstructed in accordance with a further embodiment of the presentinvention;

FIG. 4 is a block diagram illustration of a gauging apparatusconstructed in accordance with yet another embodiment of the presentinvention; and

FIG. 5 is a flowchart of an exemplary method of practicing an embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the present invention refers tothe accompanying drawings that illustrate exemplary embodimentsconsistent with this invention. Other embodiments are possible, andmodifications may be made to the embodiments within the spirit and scopeof the invention. Therefore, the following detailed description is notmeant to limit the invention. Rather, the scope of the invention isdefined by the appended claims.

It would be apparent to one skilled in the art that the presentinvention, as described below, may be implemented in many differentembodiments of hardware, software, firmware, and/or the entitiesillustrated in the drawings. Any actual software code with thespecialized controlled hardware to implement the present invention isnot limiting of the present invention. Thus, the operation and behaviorof the present invention will be described with the understanding thatmodifications and variations of the embodiments are possible, given thelevel of detail presented herein.

FIG. 1 illustrates a conventional gas gauge proximity sensor 100,according to an embodiment of the present invention. The gas gaugeproximity sensor 100 can include a mass flow controller 106, a centralchannel 112, a measurement channel 116, a reference channel 118, ameasurement channel restrictor 120, a reference channel restrictor 122,a measurement probe 128, a reference probe 130, a bridge channel 136,and a mass flow sensor 138. A gas supply 102 can inject gas at a desiredpressure into gas gauge proximity sensor 100.

The central channel 112 connects the gas supply 102 to the mass flowcontroller 106 and then terminates at a junction 114 (e.g., a gasdividing or directing portion). The mass flow controller 106 canmaintain a constant flow rate within the gas gauge proximity sensor 100.Gas is forced out from the mass flow controller 106 through a poroussnubber 110, with an accumulator 108 affixed to the channel 112. Thesnubber 110 can reduce gas turbulence introduced by the gas supply 102,and its use is optional.

Upon exiting the snubber 110, gas travels through the central channel112 to the junction 114. The central channel 112 terminates at thejunction 114 and divides into the measurement channel 116 and thereference channel 118. In one embodiment, the mass flow controller 106can inject gas at a sufficiently low rate to provide laminar andincompressible fluid flow throughout the system to minimize theproduction of undesired pneumatic noise.

A bridge channel 136 is coupled between the measurement channel 116 andthe reference channel 118. The bridge channel 136 connects to themeasurement channel 116 at the junction 124. The bridge channel 136connects to the reference channel 118 at the junction 126. In oneembodiment, the distance between the junction 114 and the junction 124and the distance between the junction 114 and the junction 126 areequal. It is to be appreciated that other embodiments are envisionedwith different arrangements.

All channels within the gas gauge proximity sensor 100 can permit gas toflow through them. The channels 112, 116, 118, and 136 can be made up ofconduits (e.g., tubes, pipes, etc.) or any other type of structure thatcan contain and guide gas flow through the sensor 100, as would beapparent to one of ordinary skill in the art. In most embodiments, thechannels 112, 116, 118, and 136 should not have sharp bends,irregularities, or unnecessary obstructions that can introduce pneumaticnoise. This noise can result from the production of local turbulence orflow instability, as an example. In various embodiments, the overalllengths of the measurement channel 116 and the reference channel 118 canbe equal or unequal.

The reference channel 118 terminates adjacent a reference probe 130.Likewise, the measurement channel 116 terminates at an adjacentmeasurement probe 128. The reference probe 130 is positioned above areference surface 134. The measurement probe 128 is positioned above ameasurement surface 132. In the context of photolithography, themeasurement surface 132 can be substrate (e.g., a wafer, a flat panel,print head or the like) or stage supporting a substrate. The referencesurface 134 can be a flat metal plate, but is not limited to thisexample.

Nozzles are provided in the measurement probe 128 and the referenceprobe 130. An example nozzle is described further below with respect toFIGS. 2-4 below. Gas injected by the gas supply 102 is emitted fromnozzles in the probes 128 and 130, and impinges upon the measurementsurface 132 and the reference surface 134.

As described above, the distance between a nozzle and a correspondingmeasurement or reference surface can be referred to as a standoff.

In one embodiment, the reference probe 130 is positioned above a fixedreference surface 134 with a known reference standoff 142. Themeasurement probe 128 is positioned above the measurement surface 132with an unknown measurement standoff 140. The known reference standoff142 is set to a desired constant value, which can be at an optimumstandoff. With such an arrangement, the backpressure upstream of themeasurement probe 128 is a function of the unknown measurement standoff140; and the backpressure upstream of the reference probe 130 is afunction of the known reference standoff 142.

If the standoffs 140 and 142 are equal, the configuration is symmetricaland the bridge is balanced. Consequently, there is no gas flow throughthe bridging channel 136. On the other hand, when the measurementstandoff 140 and the reference standoff 142 are different, the resultingpressure difference between the measurement channel 116 and thereference channel 118 induces a flow of gas through the mass flow sensor138.

The mass flow sensor 138 is located along the bridge channel 136, whichcan be at a central point. The mass flow sensor 138 senses gas flowinduced by pressure differences between the measurement channel 116 andthe reference channel 118. These pressure differences occur as a resultof changes in the vertical positioning of measurement surface 132.

In an example where there is a symmetric bridge, the measurementstandoff 140 and the reference standoff 142 are equal. The mass flowsensor 138 will detect no mass flow because there will be no pressuredifference between the measurement and the reference channels 116 and118. On the other hand, any differences between the measurement standoff140 and the reference standoff 142 values can lead to differentpressures in the measurement channel 116 and the reference channel 118.Proper offsets can be introduced for an asymmetric arrangement.

The mass flow sensor 138 senses gas flow induced by a pressuredifference or imbalance. A pressure difference causes a gas flow, therate of which is a unique function of the measurement standoff 140. Inother words, assuming a constant flow rate into the gas gauge 100, thedifference between gas pressures in the measurement channel 116 and thereference channel 118 is a function of the difference between themagnitudes of the standoffs 140 and 142. If the reference standoff 142is set to a known standoff, the difference between gas pressures in themeasurement channel 116 and the reference channel 118 is a function ofthe size of the measurement standoff 140 (that is, the unknown standoffalong a vertical (Z) axis between the measurement surface 132 and themeasurement probe 128).

The mass flow sensor 138 detects gas flow in either direction throughthe bridge channel 136. Because of the bridge configuration, gas flowoccurs through the bridge channel 136 only when pressure differencesbetween the channels 116 and 118 occur. When a pressure imbalanceexists, the mass flow sensor 138 detects a resulting gas flow, and caninitiate an appropriate control function, which can be done using anoptional controller 150 that is coupled to appropriate parts of thesystem 100. The mass flow sensor 138 can provide an indication of asensed flow through a visual display and/or audio indication, forexample, which can be done through use of an optional output device 152.

Alternatively, in place of a mass flow sensor, a differential pressuresensor (not shown) can be used. As well understood by those of skill inthe art, a differential pressure sensor is designed to detect a changein pressure as a difference between two applied pressures. Thedifferential pressure sensor measures the difference in pressure betweenthe two channels, which is a function of the difference between themeasurement and reference standoffs.

The control function in the optional controller 150 can be to calculatethe exact gap differences. In another embodiment, the control functionmay be to increase or decrease the size of the measurement standoff 140.This is accomplished by moving the measurement surface 132 relative tothe measurement probe 128 until the pressure difference is sufficientlyclose to zero. This occurs when there is no longer a difference betweenthe standoffs from the measurement surface 132 and the reference surface134.

It is to be appreciated that the mass flow rate controller 106, thesnubber 110, and the restrictors 120 and 122 can be used to reduce gasturbulence and other pneumatic noise, which can be used to allow thepresent invention to achieve nanometer accuracy. These elements can allbe used within an embodiment of the present invention or in anycombination depending on the sensitivity desired.

For example, if an application required very precise sensitivity, allelements can be used. Alternatively, if an application required lesssensitivity, perhaps only the snubber 110 would be used with the porousrestrictors 120 and 122 replaced by orifices. As a result, the presentinvention provides a flexible approach to cost effectively meet therequirements of a particular application.

Porous restrictors 120 and 122 are also used. The porous restrictors 120and 122 can be used instead of saphire restrictors when pressure needsto be stepped down in many steps, and not quickly. This can be used toavoid turbulence.

The measurement channel 116 and the reference channel 118 containrestrictors 120 and 122. Each of the restrictors 120 and 122 restrictsthe flow of gas traveling through their respective measurement channel116 and the reference channel 118. The measurement channel restrictor120 is located within the measurement channel 116 between the junction114 and the junction 124.

Likewise, the reference channel restrictor 122 is located within thereference channel 118 between the junction 114 and the junction 126. Inone example, the distance from the junction 114 to the measurementchannel restrictor 120 and the distance from the junction 114 to thereference channel restrictor 122 are equal. In other examples, thedistances are not equal. There is no inherent requirement that thesensor be symmetrical; however, the sensor is easier to use if it isgeometrically symmetrical.

FIG. 2 is an illustration of a gauging apparatus 200 constructed inaccordance with an embodiment of the present invention. The exemplarygauging apparatus 200 of FIG. 2 can be used, for example, to supplementand/or replace the measurement probe 128, shown in FIG. 1. Morespecifically, output control signals 201 produced by the gaugingapparatus 200 provide an extended air gauge reading. This extended airgauge reading is analogous to control signals output from themeasurement probe 128, and forwarded along a feedback path 154.

According to the present invention, many of the limitations ofconventional air gauge sensors can be overcome by replacing theconventional air gauge sensors with devices that use alternative sensingtechniques. The exemplary gauging apparatus 200 is one such device.

The gauging device 200, of the present invention, essentially extendsthe measurement range of conventional gas proximity sensors bymaintaining a constant gap between the sensor and a target, such as awafer surface. This constant gap is maintained by either servoing theposition of the sensor or servoing the target to reduce the sensitivityof the gauge to error, thus improving performance.

For example, although conventional air gauges are fairly accurate, theiraccuracy is restricted to relatively short distances. That is, airgauges typically have fairly short working distances, and much shortermeasurement ranges than working distances. For example, a requirementmay exist to measure a distance of 10 micrometers (μm). A conventionalair gauge sensor, however, may have an accurate measurement range ofonly 1 μm.

By using the present invention, the air gauge is maintained at aconstant gap and is restricted to measuring very miniscule changes(e.g., on the order of several nanometers) in the distance between thetarget and the air gauge. Then, for example, in one embodiment of thepresent invention, the air gauge can be moved or servoed as the distancebetween the air gauge and the target changes.

In being restricted to measuring small distances, the air gauge is onlyrelied upon to measure the miniscule changes in distance between the airgauge and the target. Another sensing device is subsequently used tomeasure the movement of the air gauge. A combiner is then used to addthe measured distance of the air gauge device with the measured distanceof the second sensing device to produce a significantly more accuratecombined measurement reading.

As noted above, the gauging apparatus 200 of the present inventionproduces a more accurate (i.e., extended) air gauge reading. This moreaccurate reading is represented by output control signals 201. Morespecifically, the output control signals 201 more accurately representthe distance between an air gauge and a target, such as a wafer surface.

In the embodiment of FIG. 2, for example, the gauging apparatus 200 canbe used to measure distances associated with a wafer 202 mounted on amovable wafer stage 204. In practice, the wafer stage 204 can bemoveable in six degrees of freedom. However, for purposes ofillustration only, the present invention will focus on measuringmovement in only two degrees of freedom, along a vertical (Z) axis to ahorizontal surface of the wafer stage 204.

The gauging apparatus 200 includes a metrology frame 206. In the presentinvention, the term “metrology frame” is used to denote an isolatedframe of reference, which can be mechanically isolated from itsassociated measurement apparatus. Conventional metrology frames includesensitive components such as interferometers and other position sensors,which are isolated from vibration and other movements within thestructure of the metrology frame. In the embodiment of FIG. 2, themetrology frame 206 includes an air gauge 208 and a sensor 210. Thesensor 210 can include an interferometer, a cap gauge, an encoder, orthe like. The sensor 210 measures a distance 211 to the wafer stage 204.

Also included in the gauging apparatus 200 is a motion generatingmachine 212, and a combiner 214. The present application is focused onmovement in two degrees of freedom, i.e., along the vertical (Z) axis.The motion machine 212 can be an actuator, a motor, a controller, or anyother device capable of producing motion. The gauging apparatus 200 isused to accurately measure a distance 216 between the air gauge 208 andthe wafer 202.

In the example of FIG. 2, the distance 216 is maintained at asubstantially constant gap. That is, the wafer 202 is desirably mountedto the wafer stage 204. During a measurement session, the distance 216can change at least slightly, for example, due to changes in topographyof the wafer 202. In the embodiment of FIG. 2, however, although thetopography of the wafer 202 may change, the air gauge 208 is maintainedin a substantially fixed position.

The wafer 202, mounted to the wafer stage 204, is moved along the (Z)axis by the motion machine 212. The purpose of the movement along the(Z) axis is to make any adjustments necessary to maintain the distance216 at a substantially constant value. That is, the motion machine 212produces drive signals 218 that move the wafer stage 204 along the (Z)axis whenever slight changes occur in the distance 216. The distance 216can be a preprogrammed based upon user requirements.

As the distance 216 changes, these changes are sensed by the air gauge208. Correspondingly, measurement signals representative of any changesin the distance 216 are communicated to the motion machine 212.

In response, the motion machine 212 produces the drive signals 218 tomove the wafer stage 204 along the (Z) axis by an amount necessary toreadjust the distance 216 to the predetermined value. At the same time,air gauge gap error signals forwarded along a feedback path 220 are alsocommunicated to the combiner 214. As the wafer stage 204 moves inaccordance with the drive signals 218, its movement in the direction (Z)is measured by the sensor 210.

The measurement by the sensor 210 of the movement (in one direction) ofthe wafer stage 204 is forwarded along a path 221 to the motion machine212. In response, the motion machine 212 produces the drive signals 218to move the wafer stage 204 back, in the opposite direction. Themovements produced by the motion machine 212 are quantified, and thisquantified value is forwarded to the combiner 214 along a path 222. Thecombiner 214 then adds the values forwarded along the paths 220 and 222to produce the combined measurement distance 201.

The combined measurement distance 201 produced by the embodiment shownin FIG. 2 can be used to increase the accuracy of the proximity of aproximity sensor, such as the measurement probe 128 of FIG. 1. In thesystem of FIG. 1, for example, the combined measurement distance 201 canbe forwarded along the path 154 as a more accurate reading of thedistance 140.

FIG. 3 is an illustration of a block diagram of a gauging apparatus 300constructed in accordance with another embodiment of the presentinvention. In the embodiment of FIG. 3, an air gauge is moved or servedwhile a target is maintained in a substantially stationary position.More specifically, in the example of FIG. 3, the gauging apparatus 300is used to measure distances associated with the wafer 202 of FIG. 2. Inthe embodiment of FIG. 3, however, the wafer 202 is mounted on asubstantially stationary wafer stage 304.

The gauging apparatus 300 of FIG. 3 can include many of the componentsused in the gauging apparatus 200 of FIG. 2. For example, the gaugingapparatus 300 includes a metrology frame 306, which comprises the airgauge 208, the sensor 210, the motion machine 212, and the combiner 214from the gauging device 200 of FIG. 2. In FIG. 3, however, the metrologyframe 304 also includes an actuator 306.

During operation, the motion machine 212 adjusts the position of the airgauge 208 to minimize the amount of any air gap errors. For example,during a measurement session, as the wafer stage 302 moves along ahorizontal direction (substantially stationary along the vertical (Z)axis), the air gauge 208 maintains a distance 308 from the wafer 202, ata substantially constant value. That is, as the wafer 202 moves along inthe horizontal direction, and changes in a topography of the wafer 202occur, the air gauge 208 is servoed along the vertical (Z) axis. Theactuator 306 moves the air gauge 208 along the (Z) axis.

As the air gauge 208 moves, this movement is sensed and measured by thesensor 210. This movement is quantified and communicated to the motionmachine 212 and the combiner 214, in the form of an air gauge gapmovement signal along a feedback path 314. At the same time, an airgauge gap error signal is communicated to the combiner 214 along anerror path 312.

The motion machine 212 then readjusts the position of the air gauge 208via the actuator 306, in order to maintain the distance 308 at asubstantially constant value. Finally, the combiner 214 combines the airgauge gap error signal and the air gauge movement signal 313 to producean extended air gauge reading 316.

The extended air gauge reading 316 can be applied to the measurementprobe 128 of FIG. 1. Particularly, the extended reading 316 can beforwarded along the path 154 to increase the overall accuracy of systemssuch as the proximity gauge sensor 100.

FIG. 4 is an illustration of a gauging apparatus 400 constructed inaccordance with yet another embodiment of the present invention. Thegauging apparatus 400 of FIG. 4 operates in a manner similar to thegauging apparatus 300 of FIG. 3. However, in the exemplary embodiment ofFIG. 4, a metrology frame 402 includes a self-gapping air gauge 404,which replaces the air gauge 208 of FIG. 3. As understood by personshaving ordinary skill in the art, self-gapping air gauges include airbearings and operate based on the principles of aerostatic andaerodynamic design.

In the apparatus 400 of FIG. 4, the self-gapping air gauge 404 acts asan air bearing to sense a distance to an object. More specifically, inthe gauging apparatus 400 of FIG. 4, the motion machine 212 and theactuator 308, shown in FIG. 3, can be eliminated. Their elimination ispossible since the movement of the self-gapping air gauge 404 isself-maintained. For example, a preload force 406 applied by a spring(not shown) facilitates automatic readjustment of the self-gapping airgauge 404.

During operation, internally produced aerodynamic forces and the preloadforce 406 cooperate to maintain the distance 308 at a substantiallyconstant value. As the air gauge 404 moves, its position is sensed by,for example, the position sensor 210, which subsequently forwards an airgauge movement signal 408 to the combiner 214. At the same time, and airgauge error signal 410 is forwarded along a feedback path 410 to thecombiner 214.

As the self-gapping air gauge 404 moves, due for example to changes inthe topography of the surface of the wafer 202, the preload force 406readjusts the position of the air gauge in an attempt to maintain aconstant air gap. In this manner, the gauging apparatus 400 of FIG. 4 isable to maintain a constant distance or gap 308 without any directfeedback from the sensor 210. The air gauge movement signal 408 and theair gauge error signal 410 are combined, within the combiner 214, toproduce an extended air gauge topography measurement signal 414.

FIG. 5 is a flowchart of an exemplary method 500 of practicing anembodiment of the present invention. In FIG. 5, the gauging apparatus isused to sense a distance to a surface of an object, as indicated in step502. Next, the gauging apparatus will measure at least one from a groupincluding a relative position of an air gauge and the relative positionof the surface of the object, as indicated in step 504. In step 506, thesensed distance and the measurement are combined to produce an extendedair gauge measurement.

CONCLUSION

The present invention provides techniques, for example, whereby theposition of a wafer substrate is controlled in a classical negativefeedback loop. Using this feedback loop, a difference between the airgauge reading and a programmable set point value can be used to keep ameasurement gap constant. Thus, while scanning a wafer, the air gaugemaintains a known constant preprogrammed distance from the wafersurface.

By using the present invention, all of the desired characteristics ofthe air gauge can be preserved, while perfectly linear readings can bemaintained. Additionally, programmability of the standoff can beimproved. The air gauge can be operated at a more favorable standoff,maximizing its performance, and useful measurement range. At the sametime, the risk of a collision between the air gauge nozzle and the wafercan essentially be eliminated.

The present invention has been described above with the aid offunctional building blocks illustrating the performance of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

Any such alternate boundaries are thus within the scope and spirit ofthe claimed invention. Persons having ordinary skill in the art willrecognize that these functional building blocks can be implemented byanalog and/or digital circuits, discrete components,application-specific integrated circuits, firmware, processor executingappropriate software, and the like, or any combination thereof. Thus,the breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art (including the contents of thereferences cited herein), readily modify and/or adapt for variousapplications such specific embodiments, without undue experimentation,without departing from the general concept of the present invention.Therefore, such adaptations and modifications are intended to be withinthe meaning and range of equivalents of the disclosed embodiments, basedon the teaching and guidance presented herein. It is to be understoodthat the phraseology or terminology herein is for the purpose ofdescription and not of limitation, such that the terminology orphraseology of the present specification is to be interpreted in lightof the teachings and guidance presented herein, in combination with theknowledge of one of ordinary skill in the art.

1. An air gauge device for use in a lithography system, comprising: areference probe configured to measure a surface of a reference surfaceand to generate a reference signal; a measuring probe configured tomeasure a surface of an object and to generate a gap error signal alonga feedback path; a sensor configured to sense a position of themeasuring probe and to generate a measuring probe movement signal; amotion machine to receive the measuring probe movement signal and thegap error signal to move the measuring probe; and a combiner configuredto receive the measuring probe movement signal and the gap error signaland to generate a combination signal therefrom, wherein the desireddistance between the measuring probe and the object is substantiallymaintained by adjusting the position of the measuring probe based on themeasuring probe movement signal and the gap error signal and wherein atopography of the object is determined based at least on a comparison ofthe reference signal and the combination signal.
 2. The air gauge deviceof claim 1, further comprising: a metrology frame, wherein the sensorand the measuring probe are coupled to the metrology frame.
 3. The airgauge device of claim 1, wherein the sensor comprises an interferometer,a cap gauge, or an encoder.
 4. The air gauge device of claim 1, whereinthe object comprises a substrate.
 5. The air gauge device of claim 1,wherein the desired distance is at least one of a constant distance anda preprogrammed distance.
 6. The air gauge device of claim 1, whereinthe combination signal is a distance measurement between the measuringprobe and the object.
 7. The air gauge device of claim 1, furthercomprising: an actuator configured to adjust the position of themeasuring probe so that the desired distance between the measuring probeand the object is substantially maintained.
 8. The air gauge device ofclaim 7 further comprising: a controller configured to generate acontrol signal; wherein the actuator is configured to adjust theposition of the measuring probe based on the control signal.
 9. A methodof operating an air gauge device by measuring a surface of an objectmounted on a stage in a lithography system, comprising: measuring adistance to a reference surface; measuring a distance to the objectusing a measuring probe; generating a gap error signal along a feedbackpath; sensing a position of the measuring probe based on the measureddistance to the object to generate a measuring probe movement signal;moving the measuring probe in response to the measuring probe movementsignal and the gap error signal, wherein the desired distance betweenthe measuring probe and the object is substantially maintained byadjusting the position of the measuring probe based on the measuringprobe movement signal and the gap error signal; generating a combinedsignal based on the measuring probe movement signal and the gap errorsignal; and determining a topography of the object based at least on thecombined signal.
 10. The method of claim 9, wherein the desired distanceis at least one of a constant distance and a preprogrammed distance. 11.The method of claim 9, wherein the adjusting comprises: generating acontrol signal based on which an actuator is configured to adjust theposition of the measuring probe.